Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near‐) surface processes that occurred on early Mars. Here, we have ...conducted a transmission electron microscope study of medium‐ and coarse‐grained (a few tens to hundreds of micrometers) Ca‐rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high‐temperature (magmatic) origin as revealed by the well‐developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.
We conducted a transmission electron microscope study of the exsolution microstructures of Ca‐rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to ...provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo40, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo40 do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h−1, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h−1. We conclude that the formation of type I and II chondrules in the proto‐solar nebula was the result of a common mechanism.
The mechanisms responsible for the formation of Fe-rich, metasomatic features in eucrites and their debris in howardites are yet controversial. In this study, secondary phases found in polymict ...eucrite NWA (Northwest Africa) 11911, howardite NWA 1664, monomict eucrite NWA 8675 and unbrecciated basaltic eucrite NWA 13269 were investigated to elucidate their origin and history of formation. Among them, NWA 11911 is the only rock that displays widespread Fe-enrichment near fractures in unequilibrated and slightly equilibrated basaltic clasts and pyroxene fragments. Fayalitic olivine veinlets with various textures, thicknesses and compositions (Fa64-82) are detected in unequilibrated pyroxenes in NWA 11911, NWA 1664 and NWA 8675. Neighbouring Fe-enriched pyroxene is also Al-depleted compared to magmatic pyroxene (Al2O3 = ∼0.3 wt%). NWA 13269 contains Ca-rich plagioclase (An97-98) veinlets associated with Cr-spinel, but apparently no Fe-enrichment in pyroxene nor ferroan olivine despite a careful search. Olivine veinlets have been also detected in equilibrated pyroxene in NWA 11911. Our observations support the fluid-assisted metasomatism scenario that involves the circulation of hydrothermal fluids throughout the fractures of eucritic material.
The formation of chondrules involved major processes in the protoplanetary disk and therefore needs to be understood. Identifying possible precursors and the conditions of their transformation into ...chondrules is an essential step. Here we investigate whether refractory inclusions (RI) can be converted into Type IA chondrule analogs by isothermal heating and dynamic crystallization experiments, and report a new constraint on chondrule peak temperatures. We prepared synthetic calcium-aluminum-rich inclusions (CAI) by sintering <20 µm An + Di + Sp powder at 1200 °C and synthetic AOA analogs from crushed <5 µm Fo gel or San Carlos olivine mixed with nuggets of synthetic CAI. We used the AOA analogs as starting materials in experiments and were able to reproduce the textures and mesostasis compositions of Type IA chondrules. However, in the charges, the olivine lacks asymmetric zonation and our mesostasis compositions show olivine fractionation trends, two differences from Type I chondrules indicating the requirement of condensation of Mg and SiO in the latter. Relict spinel is present in isothermal runs up to 1550 °C, but is totally resorbed by 1600 °C. We conclude that CAI and AOA were sintered essentially at their condensation temperatures and are appropriate precursors for chondrules. Chondrules with relict spinel must have formed at <1600 °C, much lower than their liquidus temperatures (∼1750 °C). Such peak temperatures are consistent with models of condensation during chondrule formation. In typical chondrules with no inclusions of AOA or CAI, spinel is an indicator of their near complete assimilation. Grains of spinel (sensu stricto) in chondrules are relicts of RI and constitute a largely untapped cosmochemical resource for the investigation of chondrule provenance.
Abstract Northwest Africa (NWA) 14672, the most highly shocked Martian meteorite so far, has experienced >50% melting, compatible with peak pressure >~65 Gpa, at a transition stage 6/7. Despite these ...extreme shock conditions, the meteorite still preserves a population of “large” Fe sulfide blebs from the pre‐shock igneous assemblage. These primary blebs preserve characteristics of basaltic shergottites in term of modal abundance, preferential occurrence in interstitial pores along with late‐crystallized phases (ilmenite, merrillite), and Ni‐free pyrrhotite compositions. Primary sulfides underwent widespread shock‐induced remelting, as indicated by perfect spherical morphologies when embedded in fine‐grained silicate melt zones and a wealth of mineral/glass/vesicle inclusions. Extensive melting of Fe‐sulfides is consistent with the decompression path experienced by NWA 14672 after the peak shock pressure at ~70 GPa. Primary sulfides acted as preferential sites for nucleation of vesicles of all sizes which helped sulfur degassing during decompression, leading to partial resorption of Fe‐sulfide blebs and reequilibration of pyrrhotite metal/sulfur ratios (0.96–0.98) toward the low oxygen fugacity conditions indicated by Fe‐Ti oxides hosted in fine‐grained materials. The extreme shock intensity also provided suitable conditions for widespread in situ redistribution of igneous sulfur as micrometric globules concentrated in glassy portions of fine‐grained lithologies. These globules exsolved early on quenching, allowing dendritic skeletal Fe‐Ti oxide overgrowths to nucleate on sulfides.
The formation of chondrules involved major processes in the protoplanetary disk and therefore needs to be understood. Identifying possible precursors and the conditions of their transformation into ...chondrules is an essential step. Here we investigate whether refractory inclusions (RI) can be converted into Type IA chondrule analogs by isothermal heating and dynamic crystallization experiments, and report a new constraint on chondrule peak temperatures. We prepared synthetic calcium-aluminum-rich inclusions (CAI) by sintering <20 µm An + Di + Sp powder at 1200 °C and synthetic AOA analogs from crushed <5 µm Fo gel or San Carlos olivine mixed with nuggets of synthetic CAI. We used the AOA analogs as starting materials in experiments and were able to reproduce the textures and mesostasis compositions of Type IA chondrules. However, in the charges, the olivine lacks asymmetric zonation and our mesostasis compositions show olivine fractionation trends, two differences from Type I chondrules indicating the requirement of condensation of Mg and SiO in the latter. Relict spinel is present in isothermal runs up to 1550 °C, but is totally resorbed by 1600 °C. We conclude that CAI and AOA were sintered essentially at their condensation temperatures and are appropriate precursors for chondrules. Chondrules with relict spinel must have formed at <1600 °C, much lower than their liquidus temperatures (∼1750 °C). Such peak temperatures are consistent with models of condensation during chondrule formation. In typical chondrules with no inclusions of AOA or CAI, spinel is an indicator of their near complete assimilation. Grains of spinel (sensu stricto) in chondrules are relicts of RI and constitute a largely untapped cosmochemical resource for the investigation of chondrule provenance.
CHONDRULES Hewins, Roger H
Annual review of earth and planetary sciences,
05/1997, Volume:
25, Issue:
1
Journal Article
Peer reviewed
Chondrules are ∼1-mm igneous droplets in primitive meteorites, and their
abundance suggests widespread melting in the protoplanetary disk. Chondrules
with relict unmelted grains or igneous rims ...record multi111ple melting events.
There are two main types of chondrules, type I (FeO-poor and volatile-poor) and
type II FeO-rich and approximately chondritic (solar) in composition. Type I
chondrules in the unmetamorphosed chondrite Semarkona show evidence of
evaporative loss with regard to the moderately volatile elements. Loss of S
produces much of the FeNi metal in chondrules. Though the finest grained type I
and II chondrules in Semarkona are both approximately chondritic in bulk
composition, they differ in FeO content of olivine, indicating different
precursors. Simulations suggest temperatures of chondrule formation of
1550-1900°C, with short (<1 min) heating times. Short-lived
isotopes suggest that chondrules formed relatively late, and nebular shock
waves are the current favorite heating mechanism. As chondrules were
transported to the midplane more easily than fine dust or fluffy aggregates,
they were probably important components in building planets.
Numerous geosciences samples display a multi-scale mineralogical heterogeneity for which it is challenging to obtain spatially resolved quantitative chemical data. It is the case for chondritic ...meteorites, which can contain up to 10 different phases with grain size ranging from the nanometer to the millimeter. We developed a method providing multiple physical and chemical information by advanced scanning electron microscopy (SEM), hyperspectral energy dispersive X-ray spectroscopy (EDX) and electron probe micro-analyses (EPMA). The method includes: i) infra-micrometric low-voltage EDX mapping and innovative post-acquisition hyperspectral data analysis (based on both clustering and multiple linear least square fitting) which allow phase mapping and quantification of the modal abundances; ii) EPMA of chemical end-members to upgrade the phase map into a quantified chemical map; iii) physical modeling of the EDX background, used as a proxy of the density. Density maps can be obtained with a precision of ~10%; iv) determination of the bulk sample composition by combining modal abundances, chemical analysis and density measurements.
The approach is applied to three well-known chondrites (Murchison, Paris and Orgueil), showing heterogeneous grain sizes and mineralogy. Areas of ~250 ∗ 250 μm2 were mapped with a pixel size of 250 nm to determine the modal abundances, size distribution, circularity and densities of all phases, as well as the matrix bulk compositions. Taking bulk wet chemistry data as reference, ACADEMY leads to a better match than published defocused beam EPMA measurements. We demonstrate that choosing a Fe-rich, hydrated standard (a biotite) to quantify phyllosilicate by EPMA improves the quantification by up to 10%, and we ultimately retrieve the Mg/Si ratio with a 1% precision. We called this method ACADEMY for Analyzing the Composition, the modal Abundance and the Density using Electron MicroscopY. A code was developed and was made available online so that ACADEMY can be applied to other materials.
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•The composition of micrometer-sized assemblages is difficult to quantify.•A method providing multiple physical/chemical information is proposed.•Phase and density maps are obtained by low-kV EDX hyperspectral maps processing.•We applied this method to three analytically challenging chondrites.•The obtained bulk composition is consistent with wet chemistry data.
Chondrule formation models involving precursors of granoblastic olivine aggregates (GOA) of either planetesimal or nebular origin have recently been proposed. We have therefore conducted chondrule ...simulation experiments using mixtures of 100
h-thermally annealed GOA and An
+
En to test the viability of GOA as predecessors of porphyritic olivine (PO) chondrules. Isothermal runs of less than 5
min at 1350–1550
°C result in GOA disaggregation and Fe–Mg exchange; runs of 0.5–4
h show textures superficially similar to granular and PO chondrules, but with reversely zoned olivine. Charges isothermally heated at 1550
°C for 1 and 4
h before being cooled at 10 and 100
°C/h undergo olivine crystallization and yield classical PO textures. Although most evidence of origin from GOA is erased, the cores of normally zoned euhedral crystals are relict. As ‘phenocrysts’ in Type I chondrules can be relict such chondrules could have experienced similar peak temperatures to those of Type II chondrules.
Chondrules containing GOA with olivine triple junctions resemble experimental charges heated for minutes at temperatures between 1350 and 1450
°C and Type I chondrules with subhedral to anhedral olivine plus GOA relicts resemble charges heated at the same temperatures but for longer duration. Type I chondrules with a mass of granular olivine or irregular, anhedral olivine grains in the center, and much glass nearer the margin, on the other hand, require limited heating at high temperature (1550
°C) while Type I chondrules with euhedral olivines, resemble charges heated at 1550
°C for 4
h. The majority of Type I chondrules in CV chondrites display evidence of derivation from GOA. Many finer-grained chondrules in CR and UOC on the other hand, could not have been derived from such coarse-grained precursors, but could have formed from fine-grained dustballs as stipulated in the standard paradigm. Thus, both GOA and dustballs represent viable chondrule precursors of coarser and finer-grained Type I PO chondrules, respectively.
We determined proportions of Type I (reduced) and Type II (oxidized) chondrules in ordinary chondrites (OC) and found linear relationships between chondrule abundances and chondrite bulk chemical and ...oxygen isotopic compositions. Similar relationships exist between bulk oxygen isotopic compositions of carbonaceous chondrites and modal abundances of their chondritic components (matrix, Type I and Type II chondrules, refractory calcium–aluminium-rich inclusions and amoeboid olivine aggregates). These correlations can be used to predict the bulk oxygen isotopic composition of chondrites based on their petrology. We can also define model isotopic compositions associated with each petrologic component, which are not their current actual isotopic compositions due to alteration or mixing. These compositions for refractory inclusions and chondrules plot close to a slope 1 line, consistent with refractory inclusions (RI) forming from an early
16O-rich gas, the evolution of the gas to more
16O-poor compositions, possibly involving photodissociation and subsequent ice transport, followed by chondrule formation. Our results open a new understanding of the oxygen 3-isotope space and explain the unique position of OC as well as the differences between H, L and LL chondrites. They indicate that major chemical and isotopic variations between chondritic reservoirs were established after chondrule and CAI formation. They may have some bearing on the formation of planetary reservoirs: the
Δ
17O calculated for type I chondrules is appropriate for terrestrial planet progenitors, consistent with their chemical similarity to Earth mantle.
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